The present invention relates to devices for distributing optical signals. Such devices are needed in fields such as optical communications and signal processing. They may be required to split only one input signal amongst 2.sup.n output devices or alternatively may interconnect 2.sup.n input devices with 2.sup.n output devices, n being an integer greater than 1. Known devices used for this purpose have typically been formed from arrays of 4-port optical couplers interconnected by links of optical fibres in a star network configuration. These devices require large numbers of separate components, as many as 12 different couplers being required in the case of an 8 port star network. The physical complexity of such systems makes them expensive to manufature and difficult to install in-line in optical systems. Moreover, insertion losses associated with each individual coupler have a cumulative effect which significantly degraded the performance of the network as a whole.
FR-A-2,341,873 and DE-A-2454,996 disclose a method of forming a 1 to 2.sup.n optical distributor which uses n successive stages of optical beam splitters, each beam splitter stage having one or more individual transflective beam splitters to split an optical path into two further paths, and mirrors to realign the optical paths as necessary.
Such devices are capable of splitting an optical signal equally among many outputs and of distributing intact all the components of a signal which is multidimensional in the optical regime, having spatial, colour, time and polarisation components but it uses an arrangement which requires 2.sup.(k-1) separate beam splitters components for the k.sup.th stage and associated mirrors which means the number of beam splitters elements increases rapidly with increasing n.
According to a first aspect the present invention an optical distributor has at least one optical input, 2.sup.n optical outputs, where n is an integer greater than 1, one or more plane mirror surfaces and at least one transflective means, the plane mirror surface or surfaces and the transflective means being arranged to provide one optical path between the input and each of the outputs, each path making n transitions only at the transflective means, at least two of the transflective means being portions of a single transflective surface.
A device in accordance with the present invention is more compact, and may be formed using a smaller and sometimes the minimum number of different components than with previously known arrangements and can therefore be simpler and cheaper to manufacture and install. The two transflective means may be part of the same or different beam splitter stages.
The present invention is applicable to distributors having a single input to provide a 1 to 2.sup.n distributor, which is functionally equivalent to known such distributors as previously described, as well as to distributors where the above optical input is one of 2.sup.n optical inputs, for example star couplers.
The transflective means may, for example, be formed by a transflective coating on a glass substrate or by a cube beam splitter. They have the property that light incident upon the transflective means is divided into two beams of equal power, a reflected beam and a transmitted beam. The transflective means are substantially insensitive to the presence of different polarisation components in the incident light.
The or each plane mirror surface may be formed by a conventional metal or multilayer dielectric coating on a glass substrate, by a totally reflecting prism or by any other surface arranged to reflect substantially all of the power of any incident beam.
A distributor according to the present invention having one input and 2.sup.n outputs can be arranged to n beam splitter stages each of which includes a single separate transflective surface and a first plane mirror surface arranged such that the or each beam incident on each beam splitter stage is split into a reflected beam and a transmitted beam, one of the beams passing directly to any subsequent beam splitter stage, the other of the two beams being reflected at the first plane mirror surface before passing to any subsequent beam splitter stage.
Preferably the transflective surfaces are arranged to lie at substantially 45.degree. to the incident beams and the plane mirror surface is parallel to the transflective surfaces.
This arrangement provides a 1 to 2.sup.n optical distributor using one mirror and only n beam splitter elements.
Preferably the splitter stages are arranged in order of beam splitter effective length, with the shortest stage closest to the optical input. In such a device the effective length of the beam splitter is determined by the physical length of the transflective surface.
According to a second aspect of the present invention there is provided an optical distributor having 2.sup.n optical inputs according to the first aspect of the invention, each of the transflective means being in the optical paths originating from at least two distinct inputs.
A first arrangement of such an optical distributer may comprise n beam splitter stages in which each kth beam splitter stage has 2.sup.n-k spatially separate transflective surfaces arranged between a first plane mirror surface and a second opposing plane mirror surface parallel to the first plane mirror surface.
In this case each of the 2.sup.(n-k) transflective surfaces of the kth beam splitter stage form 2.sup.k of transflective means of that beam splitter stage. The stages are not necessarily in order as will be discussed later.
In another arrangements the transflective means from different stages can be formed from a single transflective element.
In such arrangements the transflective surfaces of the beam splitter stages each act as beam splitters for beam originating from two or more inputs.
A further redundancy in transflective means can be obtained in a reflective star coupler according to the present invention as each transflective means can form the transflective means necessary for the optical path of at least four inputs.
Alternatively the transflective means of all the necessary beam splitter stages can comprise portions of a single transflective surface in which case the plane mirror surfaces include separate mirror surfaces of different lengths arranged on either side of the single transflective surface.
Preferably the beam splitters extend in the direction at right angles to a first plane containing the optical input and outputs and one or more further independent sets of optical inputs and outputs are provided in a plane or planes parallel to the first plane.
A distributor in accordance with present invention has the properties of a star-network in the case with 2.sup.n inputs and 2.sup.n outputs or a tree-network in the simpler case of a single input going to 2.sup.n outputs. It is readily constructed because all the beam splitters may be identical in form, differing only in scale for the different stages. This enables the use of a minimum number of components to achieve the desired power distribution. Since there is only one path between each input and output port, multipath propagation and the resultant degradation in signal handling cannot occur. The ratio of the device endface size to its length is 2.sup.n /(2.sup.n -1) and tends to unity for large n. The power distributor is therefore well proportioned and compact, facilitating its insertion in line in optical systems.